Method for creating a substantially uniform temperature across a plastic sheet for delivery to an appliance liner thermoforming device

ABSTRACT

A sheet of plastic to be thermoformed into an appliance liner is brought to a substantially uniform temperature, preferably to a temperature differential of less than 5° F. throughout, by directing a fluid medium upon opposing side surfaces of the sheet. In the most preferred form of the invention, the fluid medium is constituted by air which is heated, impinged upon the opposing surfaces and recirculated, with the temperature and flow rate of the air being controlled based on sensed operating parameters.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention pertains to the art of thermoformingappliance liners and, more particularly, to a method and apparatus fordeveloping a substantially uniform temperature across a plastic sheetfor delivery to an appliance liner thermoforming device.

[0003] 2. Discussion of the Prior Art

[0004] In forming liners for appliances, such as refrigerators anddishwashers, it is known to extrude sheets or billets of thermoplasticmaterial to be delivered to a thermoforming device. Following theextrusion process, the sheets will be at a temperature in the order of130° F. and are typically stacked for later delivery to thethermoforming device. At times, the sheets can remain in a given stackfor a week or more prior to actually being formed into a liner.

[0005] The temperature environment in which the sheets are stacked istypically not uniformly regulated. For instance, the sheets can beplaced in a 60° F. or less environment in the winter season andsubjected to over 100° F. temperatures in the summer. In addition, sincethe sheets can remain in the stacks for some time, the outer stackedsheets will generally reach the ambient temperature, while the innersheets will stay fairly hot. Furthermore, the outer edge portions ofeach inner sheet will assume a lower temperature than the center of thesheet.

[0006] Based on at least these reasons, the actual temperatures of thevarious sheets delivered to the thermoforming device can vary greatlywhich, in turn, can result in the production of thermoformed articleshaving varying wall thicknesses and varying amounts of “formed in”stresses. This problem is particularly significant in the field of thepresent invention wherein an extruded sheet of approximately 5 mm (0.20inches) thick is stretched to form a liner in the order of two feetdeep, with the liner walls being extremely thin, generally in the orderof less than 0.7 mm (0.03 inches). With varying temperatures across thesheet, the sheet will unevenly stretch, perhaps resulting in a linerhaving certain wall portions which are disadvantageously thin and proneto cracking.

[0007] In an attempt to compensate for potential product variations, ithas been proposed in the art to pre-heat plastic sheets to bethermoformed. For instance, U.S. Pat. No. 4,842,742 is directed tocontrolling the heating of different portions of a synthetic resin sheetdelivered to a vacuum forming device to create a refrigerator liner. Theactual purpose of the controlled heating is to provide a desireddifferential thickness in different portions of the liner. Morespecifically, an oven is provided to heat a sheet received from astacked pallet prior to delivery of the sheet to a vacuum formingdevice. The heating oven includes upper and lower platens for thedifferential heating of the sheet. Other arrangements have also beenproposed to maintain accurate temperature control in the heating of atleast one side of a plastic sheet.

[0008] Regardless of the prior attempts made in pre-heating plasticsheets, there still exists a need for a method and apparatus for forminga liner of an appliance from an extruded sheet which is delivered at asubstantially uniform temperature to a thermoforming device. Morespecifically, there exists a need for a compact and efficient device andmethod for providing a consistent and substantially uniform temperaturesheet which is to be thermoformed into an appliance liner.

SUMMARY OF THE INVENTION

[0009] The present invention is directed to a method and apparatus fordeveloping a substantially uniform temperature across a sheet ofplastic, i.e., to a temperature differential of less than 5° F.throughout, prior to delivery of the sheet to an appliance linerthermoforming device. More specifically, a fluid medium flow is directedunto opposing side surface portions of a sheet being delivered to athermoforming device from a stack of extruded sheets.

[0010] In the preferred embodiment of the invention, the fluid medium isconstituted by air and at least one blower is used in combination withat least one temperature control unit to develop the flow of air used increating the uniform temperature of the sheet. The air is directedthrough nozzles to impinge upon the opposing side surface portions ofthe sheet. In the most preferred form of the invention, the air flow isheated and recirculated, with the temperature and flow rate of the airbeing controlled based on sensed operating parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic representation of a system for producing anappliance liner in accordance with a preferred embodiment of the presentinvention;

[0012]FIG. 2 is a top view of a sheet pre-heating unit incorporated inthe system of FIG. 1;

[0013]FIG. 3 is a schematic side view of the sheet pre-heating unitshown in FIG. 2;

[0014]FIG. 4 is a perspective view of a sheet pre-heating unitconstructed in accordance with a second embodiment of the invention;

[0015]FIG. 5 is a side view of the pre-heating unit of FIG. 4, withvarious internal structural components being shown in phantom;

[0016]FIG. 6 is a top view of one module of the pre-heating unit of FIG.4, with various internal structural components being shown in phantom;and

[0017]FIG. 7 is a block diagram of a control system for the pre-heatingunit of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018] With initial reference to FIG. 1, an appliance liner formationsystem, generally indicated at 2, includes a sheet forming unit 6 whichfunctions to produce sheets to be placed in a stack 9 and a temperaturecontrol unit 12 to which the sheets are delivered from stack 9 prior todelivery to a thermoforming device 15. In the most preferred form of theinvention, temperature control unit 12 constitutes a pre-heating unitused to uniformly heat the sheets prior to delivery to thermoformingdevice 15. Therefore, for purposes of discussing the preferredembodiment, reference will be made to pre-heating unit 12. In a mannerknown in the art, thermoforming device 15 is used to create an applianceliner, such as refrigerator liner 18. The present invention isparticularly directed to the construction and operation of pre-heatingunit 12 in the overall appliance liner formation system 2. In any event,for the sake of completeness, further details of appliance linerformation system 2 will be provided below.

[0019] Sheet forming unit 6 is provided with a plastic material inlet 26which directs plastic material into an extruder 28. Extruder 28 leadsinto a housing 30 within which the extruded plastic material is forcedthrough one or more dies to form sheets which are cut and delivered outof housing 30 by means of a conveyer 32 in the form of sheets, one ofwhich is indicated at 35. In practice, the various sheets 35 are stackedfor later use, typically in an area which does not have a specificallyregulated temperature environment. As discussed above, the sheets instack 9 can be subjected to temperatures ranging from approximately 60°F. or less to temperatures of over 100° F. depending upon the season ofthe year and other temperature affecting factors. In any event, itshould be readily apparent that liners formed from sheets which varygreatly in temperature when delivered to thermoforming device 15 willalso vary in wall thickness and overall durability.

[0020] With this in mind, pre-heating unit 12 is adapted to receive eachsheet 35 and to preheat the same to a substantially uniform temperatureprior to delivery to thermoforming device 15. In the most preferredembodiment of the invention, the sheet 35 is heated to establish atemperature differential of less than 5° F. throughout the entire sheet35. Therefore, a respective sheet 35 will be taken from stack 9 andplaced upon a conveyer 39 for use in transporting the sheet 35 throughpreheating unit 12. In the preferred embodiment, conveyer 39 includesfirst, second and third transversely spaced and longitudinally extendingendless support belts 42-44 for supporting and delivering each of thesheets 35 through a passage 47 defined by an enclosure portion 50 ofpre-heating unit 12. In this preferred embodiment, the multiple endlesssupport belts 42-44 are constituted by approximately 2.54 cm (1 inch)wide rubber belts which are designed to minimize the contact area withthe sheet 35 so as to expose the entire upper surface and the majorityof the lower surface of the sheet 35.

[0021] In accordance with the present invention, pre-heating unit 12achieves an accurate and consistent uniform heating of successive sheets35 by impinging a flow of heated air upon upper and lower surfaces ofthe sheet 35 within enclosure 50. For this purpose, enclosure 50 hasattached thereto an upper blower 54 and a lower blower 55 which receivean intake of air from respective return lines 57-60. Additional detailsof the structure and function of blowers 54 and 55 will be providedbelow.

[0022] Upon exiting from pre-heating unit 12, the sheet 35 will bedirectly delivered to thermoforming device 15. In a manner known in theart, thermoforming device 15 includes a male mold member 64 and a femalemold member 65 between which the sheet 35 is positioned. In thepreferred embodiment, a vacuum-based thermoforming process is performed.Although the present invention can be utilized to form various types ofappliance liners, including liners for dishwashers, in the mostpreferred embodiment, each sheet 35 is utilized to form a respectiverefrigerator liner 18 generally provided with a peripheral front flange69 and a body portion 71 that defines an interior storage area 73.Although liner 18 can be formed from various polymers, a preferredmaterial is a high impact polystyrene (HIPS). Of course, in formingliner 18, the material of sheet 35 must be stretched in order to createthe interior storage area 73. Therefore, a sheet 35 having an initialthickness in the range of approximately 5 mm (0.20 inches) is vacuumedformed into liner 18 having a preferred wall thickness of approximately0.7 mm (0.03 inches) and a depth of approximately 60 cm (2 feet). Ofcourse, the actual thickness and depth of the liner 18 can vary withoutdeparting from the present invention and the thickness of sheet 35 willcorrespondingly change.

[0023] Reference will now be made to FIGS. 2 and 3 in describing apreferred embodiment for pre-heating unit 12. In the preferred formshown, upper and lower blowers 54 and 55 are identically constructed.Although two separate blowers are shown for directing a flow of air uponthe upper and lower surfaces of each sheet 35, it should be recognizedthat a larger, single blower unit could be provided. In any event, inthe embodiment shown, each blower 54, 55 includes an intake 77 fordrawing in a flow of air to be heated through respective return lines57-60. Blowers 54 and 55 can take various forms in accordance with thepresent invention, such as variable speed, centrifugal squirrel cagetype fans, and are readily available in the marketplace. Each blower 54,55 incorporates a heating unit 80 which, in the preferred embodiment,constitutes an electric, resistance heating element. In this matter,each blower 54, 55 draws in the air to be heated and directs the air toa respective outlet 83.

[0024] The outlets 83 of upper and lower blowers 54 and 55 lead to amanifold assembly 85. In the preferred embodiment, manifold assembly 85includes an upper manifold section 88 and a lower manifold section 89.Since upper and lower manifold sections 88 and 89 are substantiallyidentically constructed, the preferred construction for upper manifoldsection 88 will now be described in detail and it is to be understoodthat a corresponding structure for lower manifold section 89 isprovided.

[0025] Upper manifold section 88 includes an inlet 92 which directlyreceives a flow of air from the outlet 83 of upper blower 54. As shown,upper manifold section 88 includes a closed rear wall 95 and is providedwith a plurality of transversely elongated and longitudinally spacedoutlet nozzles 97. As clearly indicated in FIG. 2, each nozzle 97preferably extends transversely within enclosure 50 a distancesubstantially equal to the transverse dimension of the sheet 35. Due tothe presence of closed rear wall 95, all of the air expelled from blower54 and heated by unit 80 is forced into upper manifold section 88 andmust flow through the respective nozzles 97 onto the upper surface ofone or more sheets 35 concurrently being transported through pre-heatingunit 12. In this manner, upper manifold section 88 represents a commonenclosed zone into which the heated air from blower 54 is expelled fordistribution to nozzles 97. Therefore the actual construction of uppermanifold section 88, and manifold assembly 85 as a whole, can takevarious forms in accordance with the present invention, including simpleductwork or piping, without departing from the invention. In any event,due to the similar construction for lower manifold section 89, the lowersurface of each sheet 35 also has impinged thereon a flow of heated air.Therefore, due to the arrangement of manifold sections 88 and 89 andnozzles 97, a uniformly distributed flow of heated air impinges upon theupper and lower surfaces of the sheets 35 which, in turn, enables eachof the sheets 35 to be heated to the uniform temperature in accordancewith the present invention.

[0026] In the preferred embodiment, the sheets 35 are heated to atemperature in the order of 130-140° F. for the production of liner 18,with this temperature being above any expected ambient temperature forstack 9 to assure a heating of sheets 35 in pre-heating unit 12, with atemperature of approximately 140° F. being preferred. Of course, variousfactors directly reflect upon the ability of pre-heating unit 12 touniformly and consistently heat each sheet 35 as it passes throughenclosure 50. For instance, the air flow rate, the slot width (W) foreach of the nozzles 97, the spacing (S) between adjacent nozzles 97 andthe distance (H) from the outlet of each nozzle 97 to the respectivesurface of the impinged sheet 35 are major input parameters which havebeen optimally designed in accordance with the present invention.Extensive experimental and theoretical research has been devoted overthe years to impinging flow and heat/mass transfer resulting in optimalvalues of W and S which are as follows:

W≈0.1H & S=1.4H

[0027] Reduced scale testing in accordance with the present inventionwas initially done on a sheet having the length of 42 mm (16.5 inches),a width of 20 mm (8 inches) and a thickness of 6 mm (0.24 inches). Thearrangement of nozzles 97 were established based on the above formulasutilizing a distance of 76 mm (3 inches) from the exit of the nozzles 97to the respective centerline of the sheet which resulted in a nozzlespacing of approximately 107 mm (4.2 inches) and a nozzle width of 7.6mm (0.3 inches).

[0028] It is generally desirable in accordance with the presentinvention to make pre-heating unit 12 rather compact. In accordance withthe preferred form of the invention, it is desired to have pre-heatingunit 12 sized to simultaneously heat in a range of three to five (3-5)sheets in any given time. In addition, it is desirable to establish areasonable heating time which, in the preferred embodiment, isapproximately 40 seconds. With these established parameters and desiredoperation characteristics in mind for the reduced scale testing, a flowof heated air at approximately 140° F. was impinged upon the upper andlower surfaces of the sheet of approximately one square foot at a massflow rate of 610 CFM. Furthermore, through this testing, it wasdetermined that a static pressure range of approximately 0.7-0.9 in. H₂Owithin enclosure 50 is practical and economical for the heating of thesheets.

[0029] At this point, it should be realized that the pre-heating unitcan take various forms in accordance with the present invention. Informing refrigerator liner 18, each sheet 35 is preferably constitutedby a 122 cm×244 cm (4′×8′) high impact polystyrene sheet. To assureuniform heating of each sheet 35, the pre-heating unit would actually bebuilt on a much larger scale such as represented by the embodiment ofFIGS. 4-6. More specifically, this preferred embodiment incorporates twomodular pre-heating units 12 a and 12 b which are arranged back-to-backand secured together for use with an elongated conveyer 39. Therefore,modular preheating units 12 a and 12 b are integrated to form a singleunit, with each pre-heating unit 12 a, 12 b having associated therewithan outer enclosure 105 which is preferably made from galvanized sheetmetal and structurally reinforced by a framework 110. In the preferredembodiment, framework 110 is defined by welded square steel tubing.Within enclosure 105 is provided a layer of insulation 115, such asapproximately 2 cm (0.75 inch) expanded polystyrene (EPS).

[0030] As shown, each pre-heating unit module 12 a, 12 b includes anupper blower 54 a having an associated electric heating element 80 a fordirecting a flow of heated air into a manifold assembly 85 a that leadsto a nozzle array 97 a. A similar lower blower 55 a is also providedwith a corresponding electric heating element 80 a opening into manifoldassembly 85 a for distributing heated air to nozzle array 97 b. In thispreferred embodiment, each blower 54 a, 55 a is capable of producing anoutput of 15,000 CFM and is driven by a respective electric motor 120having a preferable power rating of 5 hp. Each heater unit 80 a isconstituted by a 15 kW unit which has an associated low pressure drop.This overall arrangement functions to provide a preferred staticpressure head of 0.85 in. H₂O. Although not clearly shown in thedrawings, each of the comers for enclosure 105 is preferably reinforcedwith steel angle iron or the like.

[0031] As clearly shown in FIG. 5, enclosure 105 can simply besubdivided by suitable, preferably sheet metal walls to define upper andlower, distinct manifold sections 88 a and 89 a of overall manifoldassembly 85 a, as well as to define ducting channels 57 a, 58 a, and 59a, 60 a which function to recirculate air in a manner directlycorresponding to return lines 57-60 in the embodiment shown anddescribed above with reference to FIGS. 1-3. In the full scaleproduction model, the distances for nozzle spacing, width and heightrelative to each sheet 35 are carried over from the established reducedscale testing. However, in this embodiment, it is actually preferred toaccommodate a total of four (4) sheets 35 simultaneously within thepre-heating units 12 a and 12 b. Also preferably incorporated as part ofthe invention is a system controller generally indicated at 128 in FIG.7. In the most preferred form of the invention, a separate systemcontroller is provided for each of the upper and lower blower/heaterarrangements. In addition, each system controller 128 preferablyincludes a PID controller 130 which receives input signals related tothe current drawn by a respective blower as indicated at 132, thecurrent drawn by a respective heater as indicated at 134 and, at 136, atemperature signal representative of the sensed temperature adjacent atleast one of the nozzles 97 as indicated by the presence of a thermistor138 (see FIG. 3 or 5).

[0032] In essence, since the individual temperatures of sheets 35 canfluctuate substantially given the particular season of the year and alsothe environment in which they are stored within stack 9, the rate atwhich each blower 54 a, 55 a is driven and the amount of currentsupplied to each heating unit 80 a can be controlled to assure thedesired output temperature of the air impinged upon each sheet 35. Inorder to reduce the influence of blower assembly heat, the target sheettemperature has been selected substantially above normal ambienttemperatures. Under conditions of high ambient temperature and highsheet temperature, it may be come necessary to mitigate the effects ofheating due to the inherent inefficiencies of the blower units. While itis recognized that a cooling coil may be placed in the airstream, suchas in the return lines or directly in the blowers adjacent to the heaterelements, it is desirable to avoid both the initial and operatingexpenses of such a chiller arrangement. In the most preferred form ofthe invention, the heat introduced by the blower units is addressed bylowering the rate at which each blower is driven. An alternative methodto mitigate the heating effects of the blower assemblies is to place amotor actuated damper on one or more of the return lines. For instance,note FIG. 3 and the presence of damper 150 which is situated so that,when fully opened, damper 150 allows free entry of ambient air into thereturn lines 57-60. Correspondingly, the PID controller would have anadditional output to control damper(s) 150 as indicated in phantom inFIG. 7.

[0033] In any event, as indicated above, it is desirable to provideheated air at a rate and temperature which will uniformly heat the sheet35, i.e., to within a 5° F. differential throughout the entire sheet 35.Knowing the desired resulting sheet temperature, the varying operatingconditions can be accounted for through each controller 130 based on theblower current inputted at 132, the current inputted at 134 and thesensed temperature provided at 136, assuming a constant speed forconveyor 39. However, conveyor speed could also constitute an input tobe further regulated by controller 130 if desired. Therefore, eachcontroller 130 will output a desired current to both a respective blower54 a, 54 b and heater 80 a to maintain the desired airflow rate andtemperature. Any problems in maintaining this operation will result inthe activation of indicator lights and/or alarms at signalling device140.

[0034] Although described with respect to a preferred embodiment of theinvention, it should be readily understood that various changes and/ormodifications can be made to the invention without departing from thespirit thereof. In particular, it should be readily recognized that theinvention deals with creating a substantially uniform temperature ofeach sheet 35 for delivery to the thermoforming device 15. Therefore,although the most preferred form of the invention utilizes a pre-heatingunit to achieve this result, the sheets 35 could also be cooled to auniform temperature for delivery to the thermoforming device 15 inaccordance with the invention. In addition, the temperature and flowrate of the air, the configuration of nozzles 97, the material and sizefor sheets 35 and the temperature control duration associated with thepresent invention can be altered depending on various factors, includingthe particular product to be formed, a desired production rate, etc.,without departing from the spirit of the invention. Furthermore,although upper and lower manifold sections with separate blowers areprovided in the preferred embodiments described, it should be recognizedthat a single, annular manifold assembly with one or more blowers couldbe utilized without departing from the spirit of the invention.Therefore, the particular construction and operation of the temperaturecontrol unit can be varied while still achieving uniform temperaturethroughout a sheet preferably by an impinging air flow regardless of theinitial temperature of the various sheets entering the temperaturecontrol unit in accordance with the present invention. In any event, theinvention is only intended to be limited by the scope of the followingclaims.

I claim:
 1. An apparatus for creating a substantially uniformtemperature across a sheet of plastic prior to delivery of the sheet toan appliance liner thermoforming device comprising: a fluid mediumhaving an associated temperature; a conveying mechanism for transportinga plastic sheet into a position wherein the fluid medium is placed incontact with opposing surface portions of the plastic sheet; and atleast one temperature control unit for regulating the temperature of thefluid medium such that the plastic sheet is brought to a substantiallyuniform temperature prior to delivery to the appliance linerthermoforming device.
 2. The apparatus according to claim 1 , whereinthe substantially uniform temperature constitutes a temperaturedifferential across the entire sheet of less than 5° F.
 3. The apparatusaccording to claim 1 , wherein the at least one temperature control unitis adapted to heat the fluid medium to approximately 130°-140° F.
 4. Theapparatus according to claim 1 , further comprising: an enclosureincluding a longitudinally extending through passage, said conveyingmechanism transporting the sheet through the enclosure; a manifoldassembly including first and second manifold sections adapted to faceopposing surface portions of a sheet transported through the enclosure;and at least one blower for developing a flow of air, which constitutesthe fluid medium, into the first and second manifold sections, with theair to be distributed to flow upon the opposing surface portions.
 5. Theapparatus according to claim 4 , wherein each of the first and secondmanifold sections includes a plurality of spaced nozzles directed towardthe opposing surface portions, with the flow of air from the at leastone blower being directed through the nozzles to impinge upon theopposing surface portions.
 6. The apparatus according to claim 5 ,wherein the nozzles extend transversely across and are longitudinallyspaced within the passage.
 7. The apparatus according to claim 5 ,wherein the plurality of nozzles are spaced approximately 76 mm (3inches) from a sheet transported through the passage.
 8. The apparatusaccording to claim 4 , wherein the manifold assembly further includesrecirculation ducting fluidly connecting the passage with an inlet ofthe at least one blower.
 9. The apparatus according to claim 8 , whereinthe at least one blower includes first and second blowers, with thefirst blower directing air into the first manifold section and thesecond blower directing air into the second manifold section.
 10. Theapparatus according to claim 9 , wherein each of the first and secondblowers develops an air flow rate of approximately 15,000 CFM.
 11. Theapparatus according to claim 4 , wherein the passage is adapted toreceive a sheet having a thickness in the order of 5 mm (0.20 inches) toform an appliance liner having a depth of approximately 61 cm (2 feet).12. The apparatus according to claim 4 , wherein the apparatus has anoperating static pressure in the range of 0.7-0.9 in. H₂O.
 13. Theapparatus according to claim 4 , further comprising, in combination: asystem controller for regulating at least one of the blower and thetemperature control unit based on sensed operating parameters of saidapparatus.
 14. The apparatus according to claim 4 , wherein the at leastone blower constitutes a variable speed blower, with said temperaturecontrol unit regulating an operating speed of the variable speed blower.15. The apparatus according to claim 4 , further comprising: a damperunit for introducing a desired amount of ambient air into said manifoldassembly.
 16. A method of forming an appliance liner comprising:extruding a sheet from a plastic material; arranging the sheet in astack of similarly extruded sheets wherein the sheet is permitted tocool; transferring the sheet to a temperature control unit; directing atemperature controlled fluid medium onto opposing side surfaces of thesheet within the temperature control unit to establish a substantiallyuniform temperature across the sheet; and delivering the sheet to athermoforming device for creating the appliance liner.
 17. The methodaccording to claim 16 , further comprising: creating the substantiallyuniform temperature by impinging the fluid medium to a temperaturedifferential across the entire sheet of less than 5° F.
 18. The methodaccording to claim 16 , further comprising: delivering the fluid mediumthrough a manifold assembly unto the opposing side surfaces of thesheet; and recirculating the fluid medium through the manifold assembly.19. The method according to claim 16 , further comprising: developing aflow of air as the fluid medium through the use of first and secondblowers, with each of the first and second blowers developing an airflow rate of approximately 15,000 CFM.
 20. The method according to claim16 , further comprising: operating the temperature control unit at astatic pressure in the range of 0.7-0.9 in. H₂O.
 21. The methodaccording to claim 16 , further comprising: extruding the sheet at athickness of approximately 5 mm (0.20 inches); and creating an applianceliner with a depth of approximately 61 cm (2 feet).
 22. The methodaccording to claim 16 , further comprising: directing the air onto theopposing side surfaces of the sheet from nozzles spaced approximately 76mm (3 inches) from the opposing side surfaces.
 23. The method accordingto claim 16 , further comprising: sensing operating parameters of thetemperature control unit; and regulating the temperature control unitbased on the sensed operating parameters.
 24. The method according toclaim 23 , further comprising: heating the sheet to a temperature ofapproximately 130°-140° F.
 25. The method according to claim 16 ,further comprising: utilizing air, supplied by a blower unit, as thefluid medium; and controlling an amount of heat added to the air by theblower unit by varying an operating speed of the blower unit.
 26. Themethod according to claim 16 , further comprising: developing a flow ofair as the fluid medium through the use of at least one blower whichdirects the flow of air into a manifold assembly; and regulating theposition of a damper unit to control an introduced amount of ambient airinto the manifold assembly.